It has been known to use a twin screw extruder as a kneader for kneading a rubber, a plastic, or the like. This twin screw extruder includes two kneading screws within a chamber having a communication in a barrel axial direction from one end side to the other end side, each of the kneading screws having a kneading rotor and segments each of which forms a flighted screw or the like. In this twin screw extruder, a material to be kneaded such as rubber or plastic is supplied through an input port at one side of a barrel into a chamber to knead the material in a kneading section including a kneading rotor arranged therein while extruding the material to be kneaded to the other side of the barrel according to a rotation of the kneading screw to obtain a kneaded mixture in a desired kneading condition.
The kneading rotor is formed by combining in an axial direction of the kneading screw a plurality of kneading rotor segments each having a spiral kneading blade and a kneading blade in parallel with the axial direction of the kneading screw. In this kneading rotor, the kneading rotor segments which are adjacent to each other in the axial direction are coupled to each other such that there is no uncontinuous portion such as a step at a junction therebetween, namely, such that the adjacent kneading rotor segments are coupled to each other smoothly in its cross section.
Now, there is a technique with regard to the above twin screw extruder which is disclosed, for example, in Patent Document 1 described below. This known technique will be explained below.
This Patent Document 1 discloses a technique as to a paired screw set (kneading screws) arranged within a chamber of a barrel as well as a kneader for rubber or rubbery composition, the kneader having a kneading blade section for kneading a material to be kneaded, at a predetermined position in an axial direction of the screw set.
The kneading blade section of the screw set of this kneader is so formed that a plurality of kneading rotors are coupled to each other in the axial direction. Each of the kneading rotors includes a spiral blade, and the spiral blade of each kneading blade section is continuously coupled to form the spiral blade of each kneading rotor blade section. The spiral blade of this kneading blade section has a screw lead of a length of more than 5 times the outer diameter of the screw set. In Patent Document 1, it is disclosed that the screw lead of the kneading blade section is set to the above length, which lowers the conveyance ability of the kneading blade section, increases the amount of the material to be kneaded being filled in the kneading section of the chamber, and enhances a kneading effect.
Also, in the screw set, the other segment member near to a downstream side of the kneading blade section has a length of more than one-half the length of the outer diameter of the screw set in the axial direction. This segment member has such a structure that a plurality of segments having blades of which top portions thereof are extending in parallel with the axis are coupled to each other such that cross sections vertical to the axis of the segments are shifted in phase to each other by 90 degrees with respect to the center of the axis.
However, according to the kneader disclosed in Patent Document 1, the amount of the material to be kneaded being filled in the chamber of the kneading section increases as the length of the screw lead of the kneading blade section becomes longer, and thus a shearing force between an inner wall of the chamber and a top blade portion of the kneading rotor (hereinafter referred to as a tip portion) becomes larger, whereas the load affecting a shaft of the kneading screw (screw set) becomes larger due to a reactive force caused by the force of the tip portion pressing the inner wall of the chamber through the material to be kneaded.
Also, as the length of the screw lead of the kneading blade section becomes longer, the spiral angle of a spiral blade becomes smaller, resulting in that the tip portion of the kneading rotor is placed leaning to a specific side within the chamber. Accordingly, a shaft of the kneading screw with a kneading rotor receives the disproportional load. The kneading screw having received the disproportional load then rotates involving torque fluctuations, thereby inducing a metal contact between the kneading rotor segment and the inner wall of the chamber with ease.
In other words, if the screw lead of the kneading blade section is made longer, the kneading screw is flexed by reception of the disproportional load, thereby raising such a problem that the metal contact between the kneading rotor segment and the inner wall of the chamber invites a shorter life (earlier wear) of the kneading rotor segment or the chamber.
In the kneader disclosed in Patent Document 1, the above other segment member near to the downstream side of the kneading blade section has such a structure that a plurality of kneading disc segments are arranged such that cross sectional shapes vertical to each of the axes are alternately shifted by 90 degrees with respect to the center of the axis, and thus the shaft of the kneading screw of these kneading disc sections will hardly be affected by the above disproportional load. However, even if such sections are adjacent to each other, the disproportional load will always affect the axis of the kneading screw because there is the kneading blade section having a longer screw lead.
(Patent Document 1)
Japanese Unexamined Patent Publication (Kokai) No. 2003-245534